The invention relates to composite insulators, consisting firstly of a core formed by a longitudinal rod of composite material, to whose ends are attached the sleeves of mounting connectors made of malleable metal, and secondly of an insulating skirt of synthetic material, provided with fins and molded onto the core.
In general, the rod is composed of fibers of glass or another substance, arranged longitudinally and joined by pultrusion, i.e., by passing them between heated jaws which crosslink the thermosetting resin surrounding the fibers.
In this method, the rod has an irregular diameter, shape, and size, so that after being cut to length it is subjected to longitudinal machining intended to confer on it a circular cross section of constant and precise diameter, allowing it to be inserted in the sleeve. Experience shows that this machining affects not only the bonding resin but the fibers as well, and creates microcracks which can be the source of breakage over time.
The rod is joined to the end sleeves by various methods. The method most often employed consists in swaging each sleeve onto the rod, using jaws composed of several parts and delimiting between them a throat with a polygonal or circular cross section. When locking radially, each of the jaw elements is subjected to a radial force causing radial tightening of the sleeve which is not uniform circumferentially, regardless of the structure of the jaws, tending to make the rod oval and sometimes delaminating its constituent fibers. As a result, the core of the insulator is rejected.
Another method consists in swaging the sleeve using radial strips arranged regularly around the sleeve, mounted so that they pivot around axes perpendicular to the longitudinal axis of the sleeve. The progressive contact of their working faces on the sleeve produces a continuous compression that permits the metal to flow lengthwise ahead of them. Here again, despite the large number of areas of application of the radial swaging force of the sleeve and variation of the magnitude of this force during the swaging operation, the locking force is not really uniform and delamination is possible.
This disadvantage is combined with the difficulty of obtaining the required degree of force for radial locking of the sleeve on the rod. An adhesion bond between machined and hence nearly smooth surfaces is involved here, so that the best grip can be obtained only with maximum locking, which means swaging the rod as well. By nature, a rod made of a composite material with a matrix that is generally an epoxy, offers excellent longitudinal tensile strength, but it also has a low resistance to transverse shear. Consequently, to prevent it from breaking transversely when the sleeve is swaged onto it, the locking rate and hence the residual locking force on the rod must both be limited. Consequently, if the ends supplied are made of a metal alloy of an unstable metallurgical nature or if the sleeves are of variable thickness, the magnitude of the residual locking force can vary in such manner that it decreases, allowing the rod and the sleeve to separate for example, under the influence of torque.